The present invention relates to a deaeration vessel as used in the paper and pulp industry. The invention relates specially to an improvement both in the outlet duct of the main stock flow and the weir used for maintaining a constant level of liquid.
The best known commercially available deaeration vessel is sold under the trade mark DECULATOR by A. Ahlstrom Corporation of Finland. Deaeration vessels are designed to remove air and other gases from cellulose fiber suspensions, and simultaneously separate other light impurities, that would otherwise be detrimental to the paper-making process.
Deaeration vessels are large vessels, into which the fiber suspension is introduced as dilute mixture (e.g. about 0.1-3%, typically about 0.4-1%, consistency). The operating principle of the vessel is to boil the fiber suspension in sub-atmospheric pressure and thus separate the gas into a gas space within the vessel and then to withdraw it from the vessel, and to allow other light impurities to be separated by movement onto the liquid surface in the vessel because of their relatively small density. The suspension is withdrawn via an opening in the bottom of the vessel, in as "deaerated" a state as possible, and directed to the paper-making machine.
In order to maximize the deaerating capability of the vessel, the vessel is evacuated by means of a vacuum pump, so that the gas is withdrawn through the vacuum pump. The deaerating capability is further enhanced by introducing the suspension via nozzles above the liquid level in the vessel, whereby the gas, if any, present in the suspension in the form of bubbles is separated even before the introduced suspension contacts the suspension in the vessel. Besides regulating the level of liquid, the overflow is used for removing any light impurities collected on the liquid surface in the vessel, so as not to allow them to reach the paper-making machine. The light impurities flow over the weir to reject and further treatment, so that the impurities are separated from the suspension as is described in U.S. Pat. No. 5,084,161, the disclosure of which is incorporated by reference herein.
Conventionally the weir is usually located near one end of the vessel, and the outlet for the suspension to be directed to paper-making machine is located at about the middle of the vessel, as shown in U.S. Pat. Nos. 3,206,917, 4,419,109, and Re. 29, 472. The tendency of the deaerated suspension to form turbulence and thereby pressure fluctuations near and at the outlet, has long been a problem. The turbulence and pressure fluctuations are caused when the direction of the flow changes from a horizontal vessel flow to a downward outlet flow. It has been determined according to the invention that the source of the turbulence is on the surface of the suspension, where a formed vortex--when of sufficient magnitude--induces a pressure pulse in the suspension to be withdrawn, or, at least entrains suspension actually meant to be withdrawn at the overflow with light impurities accumulated therein. Besides the pressure fluctuations caused by the turbulence in the flow to the paper-making machine, the turbulence causes fluctuations in the exhaust overflow that is returned to circulation after treatment, which in turn causes instability.
The long-standing turbulence problem is significant because the web-forming process of a paper-making machine is very sensitive to different oscillations caused by the flow, turbulence, waving etc. of the suspension.
The improved deaeration vessel according to the invention overcomes the above drawbacks in prior art vessels. The vessel of the invention optimizes the relative locations of the weir and the outlet for the deaerated suspension so that no detrimental turbulence is formed, so that both the quality of the suspension directed to the paper-making machine and the steady operation of the process are improved. Also, the end product, i.e. paper, encounters no quality problems due to malfunctions of the deaeration apparatus. According to the invention, the vibration of the vessel itself, due to the movement of the suspension, is also significantly reduced compared to prior art vessels.
A characteristic feature of a deaeration vessel according to the invention is that the partition wall forming the weir is located immediately adjacent (e.g. less than about 50-250 mm from) the downstream side of the periphery of the deaerated medium outlet.
The invention also contemplates a method of deaerating cellulose pulp suspension, such as one having a consistency of about 0.1-3%, typically about 0.4-1%. The method comprises the steps of: Introducing pulp suspension into the vessel first portion above the liquid level. Withdrawing gas which separates from the pulp suspension from above the liquid level. Withdrawing liquid that passes over the weir from the second portion of the vessel. And, withdrawing deaerated pulp from the first portion immediately adjacent the weir so as to minimize turbulence in and of the deaerated pulp suspension withdrawn from the vessel first portion. The method may also comprise the additional step of directing deaerated suspension toward the bottom of the weir by the weir configuration, i.e. by defining the top surface, liquid level defining, portion of the weir so that it overlies the outlet through which the deaerated suspension is withdrawn.
It is the primary object of the present invention to provide a deaeration vessel (and method of the utilization of the same) which is advantageous compared to the prior art. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.